As is well known, it is possible to turn on and off some accessories of an endothermic engine to obtain advantageous energy savings on board the vehicle. For example the water pump can be disconnected during starting transients to allow the motor to reach the rated temperature as fast as possible.
For this purpose it is possible to provide a water pump that can be connected to a power source by means of a selectively mobile friction wheel actuator that comprises a driving unit and a friction wheel connected to the driving unit. For example, the power source can be a traditional drive of accessories including a pulley rigidly connected to a driving shaft, at least a pulley connected to an accessory and a belt wound around the pulleys to allow power transfer.
In particular, the friction wheel cooperates with the back of the belt along the winding arc around the pulley of the driving shaft and is mobile by means of the actuator to selectively engage by friction a pulley not cooperating with the belt and keyed to the shaft of the water pump.
Friction wheel actuators, driven by a hydraulic linear actuator that can nevertheless be damaged by the vibrations and impacts suffered in a direction perpendicular to the axis of the stem causing reliability problems, are well known. In addition linear actuators can be very poorly controlled as for the position of the friction wheel and tend to have response times which are too long in dynamic conditions.
EP-A-1464870 discloses a friction wheel actuator having the features of the preamble of claim 1.